1. Field of the Invention
The present invention relates to a cross phase modulation suppressing device in a wavelength division multiplexing optical transmission system and an optical communication system, and more particularly to a cross phase modulation repressing device in a wavelength division multiplexing optical transmission system and an optical communication system in which signals are multiplexed so that the polarization states become orthogonal to each other between the adjacent channels.
2. Description of the Related Art
In order to cope with an increase in a communication traffic according to the recent prevalence of the Internet, a main optical fiber adopts a wavelength division multiplexing transmission system for improving the transmission capacity by passing a plurality of channels of different wavelengths within one optical fiber. In this wavelength division multiplexing transmission method, it is necessary to speed up the transmission per one channel and increase the number of multiplex channels, in order to improve the transmission capacity. Since the range of the wavelength which can be used is restricted by a limited amplifier bandwidth of an optical relay amplifier, it is necessary to narrow the channel spacing in order to multiplex more channels.
The most serious problem in the wavelength division multiplexing optical transmission with the narrow channel spacing is an effect of the cross phase modulation that is non-linear interaction between different channels. The effect of the cross phase modulation causes the waveform distortion by the interaction between difference in arrival time of transferred bits and dispersion of the optical fiber, thus deteriorating the code error rate. Since the effect of the cross phase modulation becomes stronger according as the channel spacing becomes narrower, the suppression thereof is the most important problem in pursuit of a larger capacity in the future.
As the method for suppressing the cross phase modulation, “polarization interleave multiplexing” and “method of giving a delay between each channel in a transmission line” are well known. The former “polarization interleave multiplexing” makes use of the characteristic that the power of the cross phase modulation depends on the relationship of the polarization states between the mutual signals and that the power becomes the minimum in the orthogonal polarization state. The cross phase modulation is generated in all the multiplexed channels, and the effect from a neighboring channel is generally the maximum.
In a sending end of the optical fiber, the cross phase modulation can be decreased by performing the polarization interleave multiplexing that is a method of orthogonalizing the polarization states in the adjacent channels. For example, as a reference article, there is “Y. Inada et al., European Conference on Optical Communication '99, vol. 2, p.141, 1999”.
The latter “Method of giving a delay between each channel in a transmission line” is a method of giving a delay between each channel, one or several times, between a sending end and a receiving end of the optical fiber. This is the method taking into consideration that if the relative position between each channel is always constant, the effect of the cross phase modulation is accelerated. As a concrete method, there is a method in which optical signals wavelength division-multiplexed during a transmission line are once split into every channel, and combined after passing optical fibers of different lengths, and then sent to the above transmission fibers. There is another method by use of an optical fiber grating. As its reference article, there is “G. Bellotti et al., European Conference on Optical Communication '99, vol. 1, p. 204, 1999”.
As the like technique of “method of giving a delay between each channel in a transmission line”, there is a method in which signals are split into every channel by using each arrayed optical waveguide and the accumulated dispersion caused by a dispersion slope that is a wavelength dependency of the dispersion value that the transmission fiber has, is compensated in every wavelength by using a dispersion slope compensation unit (reference article: H. Taga et al., Optical Fiber Communication Conference, PD13, 1998).
The delay added cross phase modulation suppressing device having been proposed so far, however, has no function of keeping a relationship of the polarization states between the adjacent channels during a period from the input through the output. Therefore, when it is applied to the wavelength division multiplexing optical signals which are polarization-interleave multiplexed, there is a problem such that the polarization states between the adjacent channels are not orthogonal to each other at the output end of the optical delaying device and that the suppression effect of the cross phase modulation becomes smaller by the polarization interleave multiplexing of the signals in a transmission line thereafter.